An electromagnetic signal or beam may be controlled, for example, by using a lens. Lenses may be built of transparent materials such as glass or plastic, for example. In general a lens may be configured to converge or diverge a beam of light by selecting a curvature of the lens in a suitable way. To such effect, lenses may be convex or concave, for example, wherein convex lenses typically converge beams and concave lenses cause beams to diverge. Depending on the application more than one lens may be provided, such that beams of light traverse the more than one lens. Such lens assemblies may be used to process optical beams more precisely than may be achieved with single lenses, for example to control distortion.
A Fresnel lens is a special type of lens that allows more compact lenses to be produced, using up less space and material. A Fresnel lens accomplishes this by dividing the lens into annular sections separated by discontinuities. Whereas an ideal Fresnel lens would have an infinite number of annular sections, sufficiently performing Fresnel lenses may be designed with a finite number of annular sections depending on the application. In general Fresnel lenses are used in applications with less stringent performance requirements than conventional lenses. Therefore while conventional lenses are used in photography, controlling automobile headlights can be accomplished using Fresnel lenses, for example.
A further development of a Fresnel lens is a Fresnel zone plate, which relies on diffraction rather than refraction. In general amplitude-domain Fresnel zone plates may comprise radially arranged rings that alternate between opaque and transparent, whereas a phase-domain Fresnel zone plate may comprise radially arranged rings of different material thickness. A Fresnel zone plate may be arranged in a reflect array, or reflective array, configuration wherein a phase shift field is caused between an incident and reflected electromagnetic wavefront.
A Fresnel zone plate reflective array may be constructed using phase shifter elements arranged in a suitable pattern to effect a desired phase shift field to incident radiation. Selectively activating the phase shifters produces a configurable phase shift field. A selection of phase shifter type may depend on design characteristics of the system, wherein such characteristics may comprise, for example, an operating frequency of incident radiation, tolerable insertion losses, actuation speed and reliability.